CONCAT stand for concat two string or field in the MySql.
select concat('David','Jason')
Result will be
DavidJason
CONCAT_WS do same , concating two string or field but you can pass on "Separator" to join with. its like "implode"
Syntax :: CONCAT_WS ("Separator",'String/Field',,,)
Example
select concat(",",'David','Jason')
it will join two string with seprator ","
so result will be